A high gain antenna which is mounted onboard an automobile for the reception of a satellite broadcasting, for example, is supported by a biaxial rotary drive unit. As illustrated, for example, in U.S. Pat. No. 4,725,843 issued to Katsuo Suzuki, and Japanese Laid-Open Patent Applications No. 260,204/1985 and No. 224,703/1986, such biaxial rotary drive unit generally comprises a first rotary drive system which provides a rotary drive in the elevational (or azimuthal) direction, for example, while supporting an antenna mount, and a second rotary drive system which supports the first rotary drive system and which provides a rotary drive in the azimuthal (or elevational) direction.
Considering an antenna drive unit of Az/El (azimuthal/elevational) mount type as disclosed in the cited patent and patent applications, the first rotary drive system which supports an antenna is constructed as an elevational drive mechanism (including motor) and is disposed on a turntable which is in turn supported by a second rotary drive system, constructed as an azimuthal drive mechanism. Accordingly, when the antenna is to be turned continuously in the azimuthal direction, there must be provided some electrical connection means such as slip rings for feeding power to the motor in the first drive system and for transmitting signals between the antenna and stationary electrical circuits. It will be seen that a set of sliding electrical connection means must be provided to feed power to the first rotary drive system; a second set must be provided for transmitting signals received by the antenna, and a third set must be provided for transmitting signals from position sensors associated with the first rotary system, thus requiring a total of three sets of sliding connection means. However, it will be understood that such sliding contact means are subject to a problem of a poor electrical contact caused by oscillations or abrasion.
Since the first rotary drive system which is relatively massive and including a motor, a reduction gearing or the like is disposed on the turntable, the second rotary drive system which must drive the turntable for rotation is required to supply a high output torque. This means that the object which is supported by the second rotary drive system, namely, the antenna in combination with the first rotary drive system, exhibits an increased magnitude of the force of inertia, making it difficult to achieve a rapid rotational positioning control with a high accuracy.